The special alloys industry, requires a high quality, fully dense billet for rolling various product forms. Presently high quality specialty alloy bars are generally made by hot rolling round or square billets four or five inches in diameter to finished bars in a size range of tow inches or less. Such billets are generally produced for the specialty alloys industry by continuous casting, conventional static cast ingots, and VAR (vacuum arc remelting)/ESR (electroslag remelting) processed remelt ingots.
Continuous casting is the only method currently utilized that is capable of converting the liquid steel directly into solid semifinished forms. Continuous casting skips the intermediate step of forging the ingots for rolling mill feed stock and has higher yield. The disadvantages of continuous casting include the high cost of machinery and the requirement of large tonnage of the product. Many specialty mills cannot afford the large capital outlay required to purchase continuous casting equipment. Continuous casting is also not appropriate for all alloys because of macrosegregation problems.
The conventional static cast and VAR/ESR remelt ingot processes each include the added cost of open die or rotary forging to form the billet for feed stock. Ingot cross sectional dimensions are limited under static casting due to macrosegregation. Alloy macrosegregation in ingots can be greatly reduced using VAR/ESR remelting but at an added cost.
A bottom pouring process of casting long ingots could alleviate the need for open die or rotary forging by producing a billet suitable for feed stock to a rolling mill. Bottom pouring consists of pouring liquid alloy into a vertical cast iron downpour pipe or xe2x80x9ctrumpetxe2x80x9d. The liquid alloy then flows out from the bottom of the downpour pipe and into horizontal runners attached to the base of the downpour pipe. The liquid alloy travels through the horizontal runners and flows into vertical cast iron ingot molds where the liquid completely fills the void within the mold. Such a bottom-pour set-up has the advantage of eliminating the turbulent splashing associated with normal top pouring set-ups. By eliminating turbulent splashing the bottom-pour set-up results in a smoother ingot surface.
The disadvantage to conventional bottom-pouring is discontinuous solidification that results in piping. Piping is a cavity usually found in the middle of the ingot, which results from the metal contracting as it cools in the mold. Shrinkage pipes are usually an incurable defect because the cavity formed by the pipe may be oxidized and thus the pipes will not weld shut during the rolling of the ingot. To cure the defect a portion of the ingot containing the shrinkage pipes must be cropped and returned to the electric furnace as scrap. If the defect is not cropped the ingot will produce a weakened finished steel product.
During casting, under the conventional bottom-pouring process, the horizontal runners and center down-pour usually cool too quickly. Such cooling deprives the bottom ingot portion of the liquid metal needed to fill any voids created in the solidifying upper portion of the ingot mold. As the top cools the metal contracts and voids (shrinkage pipes) form within the ingot and remain unfilled as new molten metal is prevented from entering the ingot mold. Shrinkage pipes are undesirable defects which often interfere with subsequent hot forming or remelting operations. To minimize solidification shrinkage, exothermic hot tops have been fitted into the top of the ingot molds to reverse the direction of solidification, unfortunately hot tops have met with little success. A method is needed that is capable of altering to normal solidification pattern of bottom-pouring so that fully dense ingots for feed stock can be cast.
An object of the present invention is to facilitate a solidification sequence conducive to eliminating solidification shrinkage and piping. Another object of the present invention is to form fully dense long ingots using an innovative set-up of bottom poured ingot molds.
The present invention""solidification sequence begins with the top of the ingots solidifying first. The solidification process then progressively descends down along the length of the side ingots until the ingots connect with lateral runners. The lateral runners feed the side ingot with molten metal. Solidification then proceeds from the runner to the center vertical down pour pipe, which originally received the molten metal. The metal then solidifies from the bottom of the down pour pipe to the top of the pipe. Through this process fully dense long ingots can be formed.